Rights groups and legal experts have raised a number of concerns about the targeted-killing policy.

They say the decision-making process is shrouded in mystery, and it is unclear how an individual ends up on the list of targets.

The legal regime that justifies the killings in terms of a global war against al-Qaeda has also been called into question.

US government officials rarely admit that the programme even exists.

But the leaked memo outlined some of the legal arguments the officials use to justify the policy.

In the undated memo, officials argue that al-Qaeda and the US are engaged in armed conflict, that al-Qaeda militants pose an "imminent threat" to the US, and that the US Congress authorised the president to use force against the group following the 9/11 terrorist attacks.

According to the memo, a targeted killing is legal as long as the target is a "senior operational leader" of al-Qaeda or an allied group, the suspect's capture is not feasible, the operation is approved by a senior US official, and the laws of war are observed.

The assertion that there is an armed conflict allows the US to argue that the suspects, even US citizens, do not have the same rights as they would in peacetime, such as the right to a trial.

Image caption
Anwar al-Awlaki was among three Americans killed in drone strikes in Yemen in 2011

The Department of Justice documents to be released are expected to contain fuller accounts of these legal arguments.

Unnamed officials said the papers would be made available to the House and Senate intelligence committees as of Thursday morning.

"It is critical for the committee's oversight function to fully understand the legal basis for all intelligence and counter-terrorism operations," she said.

Opinion polls have suggested that the US public broadly approves of the drone programme, particularly when it is contrasted with the possibility of a ground assault.

But the policy has come under more scrutiny since a US-born man was targeted and killed in Yemen in September 2011.

Anwar al-Awlaki was alleged to be a senior figure in a regional offshoot of al-Qaeda.

Senators are expected to ask Mr Brennan about drone strikes, the memo and the killing of Awlaki when he faces his confirmation hearing.

Meanwhile, on Wednesday the US Senate armed services committee delayed a vote on the confirmation of former Senator Chuck Hagel as defence secretary.

The move came after a group of Republican senators complained that Mr Obama's pick to lead the defence department had not been sufficiently forthcoming about his finances, specifically whether he had ever received compensation from "foreign sources".